Tin plating



Jan. 19, 1932. F. F. OPLINGER TIN PLATING Filed Feb. 61929 ,Ufff/70Wous/259 INVENTOR A TTORNEY Patented Jan. 19, 1932 UNITEDSTATES PATENTorrlcs FLOYD F. OIPLINGER, 0F PERTH AMBOY, NEW JERSEY, ASSIGNOB, BYMESNE ASSIGN- MENTS, T0 THE ROESSLER & HASSLACHEIB, CHEMICAL COMPANY, A.CORPORATION OF DELAWARE TIN PLATIN G- Application med February 6, 1929.Serial No. 837,833.

- This invention has for its object the continuous production of white,smooth and n onporous tin deposits over long periods of time and at ahigh rate of speed, with practically no production of waste materials.

I-Ieretofore alkaline tin baths have been characterized by inability tocontinuously produce the bright, smooth deposits desired in commercialwork. The majority are uneconomical in that heavy sludges, probablyconsisting in the main of small particles of metallic tin miXed withtinoxides or other tin compounds, are formed in the bottom of the bathafter continued use. Such sludges must eventually be discarded.

I have now found, that highly satisfactory deposits of tin may beproduced for extended periods of time from alkaline tin baths in whichthe analytically determined concentration of alkali metal stannate bearsa definite relationship to the analytically determined alkalinity. Allof my baths possess this definite relationship within certain limits ofalkalinity and alkali 'metal stannate concentrations, described morefully below, and will therefore plate tin satisfactorily. I have furtherfound that continuous plating will occur if the above mentioned deiniterelationship is maintained in my baths.-

One way of maintaining this relationship is to add caustic alkali whenanalysis shows the alkalinity of the bath to be low and to add alkalimetal stannate when it shows the alkalinity of the bath to be high.

Above, I have specifiedalkalinity as determined by analysis, since anyattempt at determination of alkalinity by other methods will be affectedby various factors, such as -impurities usually present in commercialalkali metal stannate, among which are free caustic, the naturalalkalinity of an aqueous The minedv analytically for similar reasons. vIn determining the alkalinity, I use thymol phthalein as an indicator,since its blue color in the presence of alkali and alkali metal stannatemixtures disappears upon the acldition `of acid, for example .ofhydrochloric or sulfuric acid, at the same time that nsoluble tincompounds begin to precipitate.

Of course, other indicators can be used instead of thymol phthaleinprovided one corrects the alkalinity so determined to give it the samevalue as that which would be 0btained by the use of thymol phthalein.Such corrections could be made on the basis of pH values of thesolution.

The alkali metal stannate content of the solution may be determined byany satisfactory known method, such as reduction of the acidifiedsolution and titration against standard iodine solution. v

Because of the above stated reasons and for the purpose of simplifyingfurther explanations of my baths, I wish wherever referring toconcentrations of caustic alkali or, specically, sodium hydroxide,whether stated in terms of normality, molality, ounces per gallon, orthe like, to be understood as referring, not to concentrations ofcaustic added as such to the bath, but to total alkalinity, asdetermined by titration, using thymol phthalein as an indicator or usingan equivalent procedure. Likewise, whenever referring to concentrationsof alkali metal stannate I wish it to be understood as referring tothose concentrations determined either by an analysis of the solutionitself or by making up the bath with a material the stannate content ofwhich is accurately known. Obviously, for a given grade of causticalkali and of alkali metal stannate, whether these are chemically pureor ofcoinmercial quality, the actual amounts which' one will use inmaking up my baths can be easily calculated. by using the ratiosobtained by dissolving weighed quantities of the two materials to aconvenient volume and comparing their concentrations thus produced withthe concentrations of caustic and stannate asvdetermined from thissolution by analysis, as given above..

The accompanying graph shows the relationship between the caustic alkaliand alkali metal stannate concentrations in my baths. The concentrationof caustic for optimum plating results, as compared with theconcentration of alkali metal stannate, is represented by the 'line B ofthe graph and may also be represented by the equation:

Caustic alkali (normalit )=0.4 times the alkali metal stannatenormality) +0.3.

In this equation "a normal solution of alkali metal stannate isconsidered as containing 1A of a mole of stannate per liter. The aboveequation may therefore be expressed as,

Caustic molality=1.6 times the stannate molality 0.3. Y

' caustic which is always present mmy baths at optimum platingcompositions, regardless of the stannate concentration.

My preferred solution and method of operation'are as follows:

Example I Sodium stannate 0.4375 mol/1. or 12:43 oz/gal. Sodiumhydroxide 1.0 mol/1. or 5.34 oz/gal. (Concentrations ot the above 2ingredients by analysis of solution.)

Anodes-Straits tin Ratio ot anode to cathode area 3 to 1 Cathode currentdenslty 20 to 60 A/SF E. M. F .0 to 6.0 volts f Temperature ot ao1ution70 to 80 C. (15B to 176 F.)

A white smooth and non-porous tin deposit will be produced in a shorttime on a metal surface, such as clean sheet steel, by using a bath ofthis kind. The anode and cathode efficiencies in the above example areapproximately 50% each, calculated on the basis of divalent tin. A

Results comparable with the above may also be obtainedwith thefollowing-bath; A

E example II s Sodium stannate 0.634 mol/1. or 18.0 oz/g'al. Sodiumhydroxide 1.3 mo 1. or 6.94 oz/gaL (Concentrations ot the above 2 ingents by analysis of solution.)

modes-Straits tin Ratio of anode to cathode area- 3 to 1 Cathode currentdensity 20 to 60 A/SF n. M. r 4.o to 6.o vom Temperature of solution -80C. (158 to 176 F.)

Other examples of my baths may be obtained by consulting theaccompanying graph wherein symbol (D denotes compositions from whichexcellent deposits were obtained; the anodes remaining clean and theanode and cathode eiliciencies being equal to within about 10%. Thesymbol a: on the graph above line A denotes bath compositions from whichspongy deposits were obtained.

Instead of making up In tin baths with alkali metal stannate itself, mayuse any tin salt which does not 'put objectionable impurities in thebath in the presence of alkali, to form the stannate in situ.

The sodium stannate concentration of my baths may vary from practicallyzero to the value at saturation of the solution with the same; goodresults being obtained between 0.07 and 0.70 moles per liter or betweenabout 2.0 and 20 oz. per gallon, provided the caustic soda contentalways bears a close relation to the stannate content as shown by thegraph. My preferred range, however, ies between about 0.25 and 0.65moles per liter, that is, about a 1 normal to 2.6 normal solution or,between about 7.0 oz/gal. and 18.5 ozs. per gallon, since experience hasdemonstrated that below this range of stannate concentration, too muchor too little caustic, to fall in the range represented between lines Aand C, is much more likely to be produced during plating as the resultof accidental variations in conditions, such as changes in currentdensity, tem erature, introduced impurities, etc. Below? oz/gal. ofsodium stannate, or its equivalent in other alkali stannate, the currenteiiiciencies are usually lower with an attendant poor quality ofdeposit. On the other hand, when the stannate concentration is greaterthan about 18.5 oz/ al., the losses produced by the adhering og some ofthe solution to the plated articles as they are removed from the bath istoo great for economical operation. Crystallization of tin salts fromsolutions at these higher concentrations has also beennoticed.

Good results may be obtained by keeping the plating solution below 85the range' of 60 to 80 C. Above 85 C. there is danger of precipitationof solids within the bath.

I have found that my tin baths operate satisfactorily with cathodecurrent densities above about 10 amperes per square foot, good depositshaving been produced at 100 A/SF. Satisfactory anode current densitieslie below 20 A/SF.

My tin plating baths may be used for lating many kinds of metal andarticu arly steel, copper, brass, lead, zinc, ca ium, cast iron and thelike. s

What I claim is 1. An aqueous electrolyte for the electro- `depositionof tin, containing 1.6 moles of caustic alkali per mole of alkali metalstannate plus an additional concentration, not dependent upon the amountof said stannate, of about 0.2 to 0.4 moles of caustic alkali per liter.

- 2. An aqueous electrolyte for the electrodeposition of tin, containing1.6 moles of caustic alkali per mole. of alkali metal stannate plus anadditional concentration, not dependent upon the amount of saidstannate, of

C., but I prefer about 0.2 to 0.4 moles of caustic alkali per nate being0.25 to 0.65 moles per liter.

3. An aqueous electrolyte for the electrodeposition of tin,containingabout 1 mole of caustic alkali and about 0.4375 moles of al: kali metalstannate per liter of solution.

4. An electrolyte for the electrodepositionl of tin, comprising anaqueous solution containing 1.6 moles of caustic soda per mole of sodiumstannate plus an additional concentration, not dependent upon the amountof stannate, of about 0.2 to 0.4 moles of caus- .tic soda .per liter.

5. An electrolyte for the electrodeposition of tin, comprising anaqueous solution containing 1.6 moles of caustic soda per mole of sodiumstannate plus an additional concentration, not dependent upon the amountof stannate, of about 0.2 to 0.4 molesof caustic soda per liter, theconcentration of sodium stannate being 0.25 to 0.65 moles per liter.

6. An electrolyte for the electrodeposition of tin, which comprises 1mole of caustic soda and 0.4375 moles of sodium stannate per liter ofsolution.

7. A process for the electrodeposition of metallic tin, comprisingconducting an electrolyzing current from a tin anode to the article tobe coatedv as a cathode through an aqueous solution of alkali metalstannate and caustic alkali whose composition is limited by a causticnormality which increases at auniform rate from 0.7 when the stannate.normality is 1, to 1.34 when the stannate lsodium hydroxide whosecomposition is limited by a sodium hydroxide normality which increasesat a uniform rate from 0.7 when the stannate normality is 1, to 1.34when the stannate normality is 2.6, the range of sodium hydroxidenormality at each normality of stannate being 0.1 less to 0.1 more thanthe values thus defined.

9. In a process for the continuous electrodeposition of tin from anaqueous solution of caustic alkali and alkali metal'stannate, the stewhich comprises regulating the compostxon of said solution so asA tomaintain 1.6 moles of caustic alkali per mole of said stannate plus anadditional concentration, not dependent upon the amount of stannate, ofabout 0.2 to 0.4 moles of caustic alkali per liter.

10. In a proccssfor the continuous electrodeposition of tin from anaqueous solution of caustic soda and sodium stannate, the step whichcomprises regulating the composition of said solution so as to maintain1.6 moles of caustic soda per mole of sodium stannate plus an additionalconcentration, not dependent upon the amount of stannate, ot

about 0.2 to 0.4 moles of caustic soda per liter.

Signed at Perth Ambo in the county of Middlesex and State of ew Jerseythis 5th day of January A. D. 1929.

FLOYD F. OPLINGER.

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